Dietary supplements and other products containing oils and fatty acids, for example omega-3 fatty acid supplements, vary significantly in concentration and purity. However, high concentration/high purity products are of indistinguishable to the naked eye from lower purity/lower concentration products at or above ambient room temperature. There exists a need for a simple method for a simple method for distinguishing low purity/low concentration oil-containing products from high purity/high concentration supplements or other oil containing products.